Fluidic electrostatic energy harvester

ABSTRACT

One embodiment of the present invention relates to a variable capacitor that operates without moving mechanical parts. In this capacitor electrically conductive electrodes are separated by an enclosed chamber filled with an electrically conductive material. The electrically conductive material can freely vary its position within the chamber. The capacitance of the device will vary as position of the conductive material changes due to external mechanical motion (ex: rotation, vibration, etc.) of the device. Other embodiments of this device are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/858,474 filed on Sep. 20, 2007, the contents of which are hereinincorporated by reference in their entirety.

FIELD OF INVENTION

The present invention relates generally to a variable capacitor intendedfor use within an energy harvesting system. The energy harvesting systemcan further be used to power a tire sensor system in one embodiment.

BACKGROUND OF THE INVENTION

Energy harvesting is a process of taking energy from a chaotic systemand converting it to a form of energy that can either be stored or usedin a controlled manner. Energy harvesting is well known in forms such ashydroelectric turbines, solar cells, wind turbines, and other similarsystems that have gained wide public exposure. These large systemsharvest relatively large amounts of energy. Recently, science has begunto look for smaller energy harvesting systems. As the power requirementsof small electronic devices have decreased, attempts have been made toharvest energy on a smaller scale and use that energy to power thedevices.

Many of these micro energy harvesting systems exploit mechanical energyand are based around MEMS or micro-machined fabricated capacitorelectrodes. The initial charging of the capacitor is provided by usingan external voltage supply or an electret material, a material which hasa built in electric field (i.e. the analogue of a permanent magnet). Themeans used to subsequently change the capacitance and harvest energyvaries among systems. For example, in the case of inertial energyharvesters, mechanical motion of the system moves a mass such that theinertia of the mass is harvested and used to increase the capacitance ofthe system.

There are two main problems associated with these devices. First, thesemovable mechanical parts are known to be a major source of failure inthese devices due to crack formation, beam break, sticking or othermechanical problems caused by the stress. Second, due to existinginfrastructure, the MEMS are often fabricated in silicon thereforecausing an inertial mass system to be limited in efficiency by thedensity of silicon.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summarypresents one or more concepts of the invention in a simplified form as aprelude to the more detailed description that is presented later and isnot an extensive overview of the invention. In this regard, the summaryis not intended to identify key or critical elements of the invention,nor does the summary delineate the scope of the invention.

One embodiment of the present invention relates to a variable capacitor,where an enclosed chamber and an insulating layer separate theelectrodes of the capacitor. Within the enclosed chamber is a conductivematerial that can move freely within the enclosed chamber as the deviceundergoes changes in its orientation and/or position. So long as theconductive material is in contact with one of the electrodes aconductive path is created between the electrode and the conductivematerial, therefore essentially changing the size of the contactingelectrode as the conductive material moves within the enclosed chamber.As the size of the electrode changes, the distance between electrodes ofthe variable capacitor changes and the capacitance changes. Otherembodiments of the device and methods are also disclosed.

The following description and annexed drawings set forth in detailcertain illustrative aspects and implementations of the invention. Theseare indicative of only a few of the various ways in which the principlesof the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the basic structure of an embodiment of the disclosedvariable capacitor;

FIG. 1B shows the variable capacitor of FIG. 1A with the addition of anelectret material placed between the electrodes;

FIG. 2A shows the electrodes of a parallel plate capacitor at a fixeddistance with charges included;

FIG. 2B shows the electrodes of a parallel plate capacitor with aconductive material between the plates and the effective distancebetween plates with charges included;

FIG. 3A shows an electret material with charges and electric fieldlines;

FIG. 3B shows an electret material as part of a parallel plate capacitorwith charges and electric field lines;

FIG. 4A shows the variable capacitor of FIG. 1B with the addition ofconductive material along the walls of the enclosed chamber;

FIG. 4B shows the variable capacitor of FIG. 2A with additional posts ofconductive material normal to an electrode;

FIG. 5A-5C shows a sequence corresponding to the rotation of thecapacitor of FIG. 3B;

FIG. 6 shows an example of an energy harvesting system circuit;

FIG. 7A shows an example of a battery system containing an energyharvester;

FIG. 7B shows an example of a tire sensor system containing an energyharvester;

FIG. 7C shows an example of a tire monitoring system containing anenergy harvester; and

FIG. 8 shows an exemplary methodology by which mechanical energy isconverted to electrical energy.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to theattached drawing figures, wherein like reference numerals are used torefer to like elements throughout, and wherein the illustratedstructures and devices are not necessarily drawn to scale.

Referring now to FIG. 1A, the basic structure of the variable capacitorcomprises two electrodes 100 and 101, separated by an insulating layer102 and an enclosed chamber 103. The enclosed chamber can be fabricatedof various materials and of differing size according to variousembodiments. Within the enclosed chamber 103 is contained a conductivematerial that can change its position freely when a force is appliedthereto (e.g., gravity, acceleration, centrifugal motion, etc.). In oneembodiment, the force applied to the conductive material will result ina change of position or orientation of the variable capacitor. Theconductive material, for example, in one embodiment could be acombination of fluid 104 and gas 105, so long as the fluid is conductive(e.g., fluid metal, ionic solutions, fluids highly doped with metallicparticles, etc.) and the gas allows the fluid to move freely within theenclosed chamber. Through use of different conductive materials withvarying dielectric constants, different amounts of conductive material,and different chamber sizes, the minimum and maximum capacitance valuesof the device can be adjusted. For the purpose of the figures anddescription in this disclosure the conductive material will refer to aconductive fluid and gas mixture, but the invention is in no way limitedto these materials. In one embodiment, the insulating layer 102 keepsthe electrodes of the device from electrically shorting together. Theinsulating layer can be placed at any distance between the electrodes invarious embodiments. It can be placed abutting an electrode or in themiddle of the enclosed chamber so long as no conductive path is formedbetween the two electrodes. The device of FIG. 1A also has externalconductive connections 106 on the outside of the conductive electrodesto facilitate connection to external circuits.

In another embodiment of the disclosed invention an additional electrodecan be added to the device. In such an embodiment the normal of theadditional electrode would be perpendicular to the existing electrodes.An additional enclosed chamber comprising a conductive fluid could alsobe placed between the additional electrode and the existing electrodes.The inventors have contemplated the addition of multiple electrodes andenclosed chambers to the variable capacitor device in this manner.

The basic operation of this device is based upon electrodynamicsprinciples that can be understood by considering a parallel platecapacitor as shown in FIG. 2A comprising two electrodes 100 and 101spaced apart at a fixed distance 200. Each electrode of this capacitorinitially contains an uneven number of positive and negative charges onthem. As the amount of positive and negative charge is different, eachelectrode will have an overall charge of either a positive or a negativevalue. Due to its overall charge, each electrode will contribute to anelectric field and since charges in an electric field have a potentialenergy due to their position, the system contains a certain amount ofenergy due to the position of the charges relative to each other. Theenergy of the system will increase as the charges get closer to eachother and their potential energy increases. The difference in potentialenergy between the two capacitor electrodes is given by the equation

ΔV=−∫E·ds

where E is the electric field, and ds the line element between thecharge and the field source. For the sake of energy harvesting the signof the energy change is unimportant, however, what is relevant is thatthe system gains potential energy within an electric field and loses itspotential energy in the form of current.

If there is no conductive path between the electrodes of the parallelplate capacitor, as in FIG. 2A, the charges cannot move closer to eachother then the distance between the electrodes and the charges of thesystem have a constant potential difference between them. On a morepractical level, these charges, separated by a distance, will alsocorrespond to a capacitance. The capacitance is proportional to theamount of charge and the distance between opposite charges. As thedistance between the electrodes (charges) changes, the capacitance (andpotential energy) of the system will change.

Referring now to FIG. 2B, the parallel plate capacitor of FIG. 2A isshown with a conductive fluid 104 between the electrodes 100 and 101.The conductive fluid 104 is in contact with the bottom electrode 101,thus forming a conductive path by which charges can move. Coulomb's lawtells us that electrical charges of opposite polarity will be attractedtoward each other by a force inversely proportional to the distanceseparating the charges. Therefore, since the charges now have aconducting path by which they can get closer, they will. Essentially,this means the bottom electrode 101 now includes the fluid 104 andtherefore the bottom electrode 101 effectively moves closer to the topconductive electrode 100. The effective distance between the topelectrode 100 and the bottom electrode 101 and 104 is reduced to 201from 200, changing the capacitance (and energy) of the system. Forsimplicity's sake, for the rest of this detailed description, theconductive electrode will comprise the solid material electrode 101 andconductive fluid 104 if it is in contact with the electrode. Since thecapacitor can be easily integrated into a circuit, it is a simple stepto harvest such a change in capacitance (energy) as described above.

Since fluids are a viscous substance, in reality the distance betweenelectrodes of such a system as FIG. 1A would be easily changed.Mechanical energy from outside of the system will be applied to move theentire system. This will result in a change of the fluid (and charges)changing its position relative to the electrodes and thus changing thedevice capacitance (energy). In this way mechanical energy of motion isconverted to electrical energy.

The design of the variable capacitor of FIG. 1A is elegant in that itdoes not require any moving structural parts. This leads to a number ofadvantages over more complicated designs, including easier fabricationand high reliability. These advantages are especially evident whencompared to existing energy harvesting technology such as apiezoelectric or an inertial based system. Piezoelectric based systems,while simple in geometry, rely upon constant stressing of the materialto generate a charge. This constant stressing can lead to stressfractures within the material. Inertial based systems are much morecomplicated and difficult to fabricate. They also have increased riskdue to wear and tear of moving parts. The simple geometry and easyfabrication of the disclosed variable capacitor allow for applicationsin MEMs and such alternatives are contemplated as falling within thescope of the invention.

FIG. 1B shows an additional embodiment of the basic structure of thevariable capacitor. This embodiment comprises an electret material 107between one of the electrodes 100 and the insulating layer 102. Theconductive fluid 104 contained within the enclosed chamber 103 is alsoshown. While the embodiment of FIG. 1B shows the electret material 107between the insulating layer 102 and the electrode 100, the electretmaterial 107 can be placed anywhere between the electrodes 100 and 101,without compromising the functionality of the device. Such alternativesare contemplated as falling with the scope of the invention. Theelectret 107 is added as a way to generate an initial charge onto thevariable capacitor. In one embodiment, it could be used as analternative to initially charging the variable capacitor with anexternal power source.

Electret materials are materials that have a quasi-permanent electricfield. They are the electric equivalent of a permanent magnet. FIG. 3Ashows an electret material 300 with charges 302 and 303 and electricfield lines 301. These materials can be produced in a number of ways,but the basic idea is to subject a dielectric material to a strongelectric field in the presence of heat or light. Upon cooling, thedipoles of the material will align and the material will retain anelectric field. The longer the exposure and the stronger the field thegreater the polarization of the electret material will be. The firstfoil electret material was made of metallized Teflon, but a wide varietyof materials are currently being researched for their potential aselectret materials including ceramics, polymers, etc. Any such electretmaterial may be employed in conjunction with the invention.

FIG. 3B shows the electret material 300 of FIG. 3A added between theelectrodes 100 and 101 of a simple parallel plate capacitor. This figureis added to show the effect an electret material 300 has when addedbetween the electrodes of a capacitor. Positive and negative charges areshown in the figure. The fixed field of the electret material will drawthe negative charges of the capacitor to the top electrode 100 and thepositive charges of the capacitor to the bottom electrode 101. Thiscreates an initial charge imbalance between the electrodes of thecapacitor in the same way that an external power sources would.

In an additional embodiment of the invention, the electret materialcould be used as the insulating material. In such an embodiment theelectret material would retain the same position within the device,between the electrodes.

FIG. 4A shows another embodiment of the variable capacitor of FIG. 1B.In FIG. 4A an additional conductive material 400 is added along theinside walls of the enclosed chamber 103. Alternatively, the walls ofthe enclosed chamber 103 can be made from the conductive material, solong as no conducting path exist between the electrodes 100 and 101(e.g., an insulating layer 102 is between the electrodes 100 and 101from each other). This additional conductive material 400 iselectrically connected to an electrode 101 in contact with the enclosedchamber 103. It is added to ensure contact between the conductive fluid104 and the electrode 100 as the device undergoes mechanic motion.Without conductive material 400 extending into the enclosed chamber 103electrical contact, in some cases, can be lost between the electrode 100and the conductive fluid 104 as the device rotates or vibrates. Thisconductive material 400 can be, but is not limited to, the same materialas the electrodes 100 and 101 of the variable capacitor.

FIG. 4B shows the device of FIG. 4A, in one embodiment, with additionalposts 401 of conductive material normal to the electrode in contact withthe fluidic chamber and with the additional posts 401 extendingsubstantially into the enclosed chamber 103 for better electricalcontact with the conducting fluid 104. The posts 401 can extend for anydistance into the enclosed chamber 103 but they do not extend throughthe insulating layer 102 so as to create a conductive path between theelectrodes 100 and 101 of the variable capacitor. Insufficient extensionof the posts 401 may lead to loss of contact between the conductivefluid 104 and electrode 101. These posts 401 can be used separate from,or in conjunction with, the conductive material 400 on the sidewalls. Ifused separately, they will serve the same purpose as the conductivesidewalls 400, to ensure contact between the electrode 101 and theconductive fluid 104. The resistance between conductive posts 401 andconductive fluid 104 will be a function of the surface area of theconductive posts 401 in contact with the conductive fluid 104. To reducesuch resistance, a large number of posts can be used in one embodiment.If the posts 401 are used in conjunction with the conductive sidewalls400, the posts 401 serve to increase the contact between the conductivefluid 104 and electrodes 101 and therefore further decrease theresistance between the conductive fluid 104 and the electrodes 101.These posts of conductive material can be, but are not limited to, thesame material as the electrodes or sidewalls of the device.

FIG. 5A-5C show an example of how the capacitance of the system changesas the device rotates. For this example capacitance will be calculatedthrough use of the equation

C=½ε_(eff) A/d

where A is the area of the conductive plates and d is the effectivedistance between the electrodes 100 and 101. The distance between theelectrodes 100 and 101 changes as the fluid 104 within the device moves.FIG. 5A shows the device at a 0 degree rotation, FIG. 5B at a 90 degreerotation, and FIG. 5C at a 180 degree rotation. The capacitance will bedependent upon a number of factors in addition to the position of thefluid, including the effective dielectric constant of the materialsinside of the chamber 103 and the amount of fluid added to the variablecapacitor. For simplicity sake it will assumed that the chamber 103 hasa length and width of “a” (FIG. 5A-5C show a cross section of the devicesuch that a would equal twice the value of 504) and that the combinedthickness of the insulating 102 and electret material of “δ” (FIG. 5A-5Cshow only an insulating layer, it can be assumed that the device isexternally biased). It will also be assumed that the volume of theconductive fluid 104 in the enclosed chamber 103 is half the volume ofthe chamber. For FIG. 5A the effective distance 500 between electrodes100 and 101 is a/2+δ and the area of the electrodes is a². This gives acapacitance of ε_(eff)a²/(a+2δ). In FIG. 5B the system is rotated by 90degrees and the conductive fluid has shifted its position. In thissituation the capacitance of the system is a combination of thecapacitance due to the effective distances 501 and 502, but since δ<<aand the capacitance increases as the inverse of the distance, thecontribution of the effective distance 502 can be ignored since thecontribution due to 501 will dominate. Taking this into consideration,the area of the electrodes will be a^(2/)2 and the distance 501 betweenelectrodes 100 and 101, is δ. This gives a total capacitance ofapproximately ε_(eff)a^(2/)4δ. In FIG. 5C the system is again rotated by90 degree with respect to FIG. 5B and the conductive fluid has shiftedposition again. The area of the electrodes will be a² and the effectivedistance 503 between electrodes 100 and 101, is δ. This gives a totalcapacitance of approximately ε_(eff)a^(2/)2δ. From this example it canbe seen that as the device rotates the capacitance changes fromε_(eff)a²/(a+2δ) at a 0 degree rotation, to about ε_(eff)a^(2/)4δ at a90 degree rotation, and to about ε_(eff)a^(2/)2δ at a 180 degreerotation. Mechanical vibrations will similarly change the position ofthe fluid relative to the electrodes, therefore also changing thecapacitance of the device with external motion.

FIG. 6 shows an example of an energy harvesting circuit within which thevariable capacitor of this disclosure can be employed. This systemcomprises two separate circuit loops 600 and 602. There is also a center“loop” 601, but this loop only has a variable capacitor 604 and aparasitic capacitor 605 added to correctly account for the parasiticcapacitance of the circuit and can be counted as a single capacitor forpractical purposes. The left loop 600 has a voltage source 608, a firstswitch 603, and the variable capacitor 604. The variable capacitor 604is the device by which energy is converted from mechanical energy toelectrical energy through a change in the position of the conductivefluid within the capacitor. This loop 600 acts to initially charge thecapacitor through use of the voltage source 608. When the variablecapacitor 604 is charged the switch 603 can be opened and loop 600 isdisconnected. If an embodiment of the invention using an electretmaterial is employed, this loop 600 is unnecessary as the charging ofthe variable capacitor 604 is done through the electret material.

The right most loop 602 of the circuit has the variable capacitor 604, asecond switch 606, and a storage capacitor 607. This loop 602 is used tostore energy from the variable capacitor 604 in the storage capacitor607 and therefore to save converted mechanical energy as electricalenergy. The energy the device is able to harvest is dependent upon thesize of the enclosed chamber, the dielectric constant of the conductivematerial in the chamber, and the amount of material in the chamber.

An exemplary operation of the circuit of FIG. 6 comprises three phases:a pre-charge phase, a harvesting phase, and a recovery phase. Forvariable capacitors not comprising an electret layer, operation of theenergy harvesting circuit begins with a pre-charge phase. The pre-chargephase begins when the capacitor plates are at a minimum spacing. Thefirst switch 603 is closed and the second switch 606 is open. Theclosure of the first switch 603 connects the voltage source 608 to thevariable capacitor 604. The voltage source will cause a potentialdifference across the variable capacitor 604 to equal the potentialdifference of the voltage source. For variable capacitors comprising anelectret layer the potential difference is provided by the electretlayer and connection to the voltage source 608 is not necessary. Oncethe capacitor has reached the potential difference of the voltagesource/electret, the harvesting phase begins. As the energy harvestingsystem 609 undergoes physical motion, the capacitance of the variablecapacitor 604 will change as described above. Through motion, thecapacitance will decrease. Since the voltage is held constant by thevoltage source 608, the change in capacitance of the variable capacitor604 drives charge into the discharge capacitor 607. When the variablecapacitor 604 reaches a minimum capacitance value the switches, 603 and606, are opened. Subsequent physical motion returns the variablecapacitor plates to their minimum space. The return to minimum spacecauses a drop in the voltage as the charge is held constant in theisolated capacitor.

FIG. 7A shows the application of an energy harvesting system 700 as ameans to charge a battery 701 according to one embodiment. In thisfigure the energy harvesting system 700 comprises the variable capacitorand through conversion of mechanical energy to electric energy, willcharge the battery 701. Through use of a battery 701 to store theharvested energy, the energy harvesting system 700 can be applied to awide range of applications.

FIG. 7B shows the battery system 702 of FIG. 7A 702 attached to a sensormodule 703. In one embodiment, the battery system 702 and sensor module703 would both be attached to a tire. The sensor module 703 isconfigured to acquire one or more tire parameters (e.g., a tirepressure). The use of the disclosed variable capacitor as a part of thissystem would be advantageous due to the nature of the motion of thetires, as the example of FIGS. 5A-5C showed. That is, as the tirerotates the energy harvester 700 is rotated, thus charging a localcharge store associated with the sensor module 703. The local chargestore is then used to power the sensor module 703, thereby avoidingbattery replacement.

FIG. 7C shows the tire sensor system 704 of FIG. 7B integrated into atire monitoring system 707. The energy harvesting and battery component702 of FIG. 7B is now additionally used to power a small RFcommunication device 705 (e.g., transmitter, transceiver). In thisembodiment the sensors 703, battery system 702, and communication device705 are attached to each tire of the vehicle. The communication device705 relays tire sensor readings taken by the tire sensor module 703 to acentral processing unit 706 that monitors systems within the car andnotifies the driver when sensor readings are outside of a predeterminedlimit. In one embodiment the central processing unit is located withinthe car. In another embodiment the sensor module relays data to acentral processing unit remotely located from the automobile and theresults of the data are communicated to the automobile wirelessly (e.g.,through a mobile phone network).

FIG. 8 shows an exemplary methodology 800 by which mechanical energy isconverted to electrical energy. While the method 800 is illustrated anddescribed below as a series of acts or events, it will be appreciatedthat the illustrated ordering of such acts or events are not to beinterpreted in a limiting sense. For example, some acts may occur indifferent orders and/or concurrently with other acts or events apartfrom those illustrated and/or described herein. In addition, not allillustrated acts may be required to implement one or more aspects orembodiments of the disclosure herein. Further, one or more of the actsdepicted herein may be carried out in one or more separate acts and/orphases.

The method 800 starts by placing electrical charges on the electrodes ofa capacitor at 802. In one embodiment such a capacitor could have twoelectrodes, while in other embodiments the variable capacitor couldcomprise additional electrodes. Charges are placed on the electrodes ofthe variable capacitor at 802. For example, charges can be placed byinclusion of an electret material in the device or through attaching thedevice to an external power source. Other alternatives for placingcharges onto the electrodes have also been contemplated by the inventor.

At 804, an external force acting on the device changes the size of theelectrodes. The external force can be, but is not limited to, rotationof the device, vibration of the device, or linear acceleration of thedevice. In one embodiment the size is changed by enclosing a conductivematerial within the device such that the conductive material will changeits position relative to the electrodes as the device undergoes externalmotion. Other alternatives have also been contemplated by the inventorfor methods by which the size of the electrodes may be changes. As theelectrodes change size, the capacitance of the device will changeaccordingly.

The capacitance change will drive charges from the variable capacitorwhich can then be stored in a local charge store. In one embodiment thelocal charge store could be a storage capacitor. In a differentembodiment the local charge store could be a chemical battery. Thestorage of generated energy is not limited to these embodiments and canby achieved through other additional means.

Although the invention has been illustrated and described with respectto one or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component or structurewhich performs the specified function of the described component (e.g.,that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary implementations of the invention. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in either the detailed description and the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

1. A tire sensor system, comprising: a variable capacitor comprising twoelectrically conductive electrodes, wherein at least one electrode isadapted to change size as the capacitor undergoes physical motion,thereby resulting in a change of capacitance; an energy storage circuitconfigured to utilize the change in capacitance to store electricalenergy; a tire sensor module configured to use the stored electricalenergy; and a mount apparatus configured to mount the variable capacitorand sensor module onto a tire such that the variable capacitor willundergo physical motion as the tire moves.
 2. The tire sensor system ofclaim 1, further comprising a battery coupled to the energy storagecircuit and the tire sensor module.
 3. The tire sensor system of claim2, further comprising a transmitter or transceiver, coupled to thebattery and the tire sensor module, configured to wirelessly transmitdata.
 4. The tire sensor system of claim 3, further comprising a centralprocessing unit configured to receive the data from the transmitter ortransceiver.
 5. The tire sensor system of claim 1, further comprisingone or more electrically conductive posts electrically coupled to andnormal to one of the electrodes.
 6. The tire sensor system of claim 5,wherein the conductive post and the conductive inside walls comprise thesame conductive material.
 7. A method for converting mechanical energyto electrical energy, comprising: placing charges on two electrodes of avariable capacitor; generating an electrical energy through changing thelocation of the charges on the electrodes by changing the size of atleast one electrode; and storing the generated electrical energy.
 8. Themethod of claim 7, wherein the size of the electrodes is changed byrotating the variable capacitor or vibrating the variable capacitor. 9.The method of claim 7, wherein the charges are placed on the electrodesof the variable capacitor by connecting the electrodes to a voltagesource.
 10. The method of claim 7, wherein the charges are placed on theelectrodes of the variable capacitor by including an electret materialbetween the electrodes.
 11. The method of claim 7, wherein the resultantenergy is stored in a local charge store.
 12. An energy harvestingsystem, comprising: means for placing charges initially on theelectrodes of a variable capacitor; means for changing a size of theelectrodes of a variable capacitor through an external motion of thevariable capacitor thereby resulting in a change in capacitance; andmeans for converting the change in capacitance to stored electricalenergy.
 13. The system of claim 12, further comprising means for storingenergy associated with a change in capacitance of the variablecapacitor.